1)
Why does it say "Lachen"?
1.
Malbim: Because they did not accept My Mitzvos [I will give to them opportunity to stumble, and all will perish].
2)
Why does it say "Avos u'Vanim Yachdav Shachen v'Re'o"?
1.
Radak: The father does not warn his son if he knows [that his son sins], and vice-versa, and a man does not warn his neighbor, nor his colleague with whom he does business or has a partnership. Therefore, all will perish, for each desires his friend's evil.
